The present invention relates to a function generator suitable for temperature compensation of crystal oscillation frequency and a temperature compensated crystal oscillator using the function generator.
Various electronic equipments are required not only to be compact and light but also have high reliability and high accuracy these days. In such a background, a crystal resonator is widely used for generating a clock signal or the like in a large number of electronic equipments. The oscillation frequency of a crystal oscillating circuit using a crystal resonator is desired to be highly stable particularly against variation of the ambient temperature. An AT-cut quartz resonator is now the most commonly utilized as the crystal resonator.
It is known that the oscillation frequency of a crystal oscillating circuit using a crystal resonator is largely varied in accordance with the variation of the ambient temperature Ta when the temperature is not compensated. For example, the proportion of an oscillation frequency Fa (at the ambient temperature Ta) to a reference frequency Fr (at a reference temperature Tr) is varied by several tens ppm in accordance with the variation of the ambient temperature Ta ranging between −30° C. and +80° C. Also, the reference frequency Fr fluctuates. Such variation and fluctuation of the oscillation frequency can be a significant problem in an electronic equipment with high accuracy. Accordingly, there is a demand for a crystal oscillating circuit with a more stable oscillation frequency. For example, it is necessary to suppress the variation in the frequency proportion Fa/Fr to 2.5 ppm or less and the fluctuation of the reference frequency Fr to 0.3 ppm or less.
In the electronic equipment with high accuracy, therefore, temperature compensation for the crystal oscillation frequency is generally effected. For example, the crystal resonator is connected with a variable capacitance diode in series, and a compensation voltage in accordance with the ambient temperature Ta is applied to the variable capacitance diode.
In a conventional technique, where N is 1 or a larger integer, a constant current unaffected by the ambient temperature Ta is made to flow to a series circuit of N diodes, a current in proportion to the difference between the ambient temperature Ta and the reference temperature Tr is made to flow to a series circuit of N+1 diodes, and a difference between voltages generated in the above-described circuits is applied between the base and the emitter of an output transistor. Thus, a current in proportion of the N+1th power of Ta−Tr is generated in the collector of the output transistor. If, assuming that N=2, a current in proportion to the cube of Ta−Tr is generated and a compensation voltage to be applied to a variable capacitance diode is generated from the current, third order temperature compensation can be achieved (see U.S. Pat. No. 5,719,533).
For equipment requiring even higher accuracy with respect to temperature compensation, a high order control technique such as a fourth-order or fifth-order control technique is necessary (Japanese Unexamined Patent Publication No. 2003-8386).
To form a cubic function generator in the conventional technique in which a diode series is used, a supply voltage which can drive a series circuit of three diodes is necessary. Moreover, a driving voltage for four diodes and a driving voltage for five diodes are required for forming a fourth-order function generator and a fifth-order function generator, respectively.